Powder feed for electrostatic printing system with an electric field free chamber



Sept. 20, 1966 R. w. MELMON 3,273,496

POWDER FEED FOR ELECTROSTATIC PRINTING SYSTEM WITH AN ELECTRIC FIELD FREE CHAMBER Filed Aug. 28, 1964 llll A\R VACUUM SOURCE E EEO o W DW V WWE m H R THQ :5

o o o g 74 OUT //\/VEN7O/? lP/CHARD W/WELMON B /fmwe My A 770 RNEY United States Patent POWDER FEED FOR ELECTROSTATIC PRINTING SYSTEM WITH AN ELECTRIC FIELD FREE CHAMBER Richard W. Melmon, San Francisco, Calif., assignor, by mesne assignments, to Cracker-Citizens National Bank, as trustee Filed Aug. 28, 1964, Ser. No. 392,717 4 Claims. (Cl. 101114) This invention relates to electrostatic printing systems and more particularly to improvements therein.

An electrostatic printing system has been described in Patent No. 3,081,698 wherein a screen having apertures therein only in a shape of an image desired to be printed is placed adjacent to an opposing backing electrode. A source of potential is applied between the screen and the backing electrode for the purpose of establishing an electric field therebetween. Electroscopic powder partio'les which have a physical size smaller than those of the screen apertures are urged through the screen apertures by means, which usually is a brush, into the electric field. There the particles are carried by the reaction of the charge in the electric field to the backing electrode where they remain forming the same image as is defined by the apertures of the screen. Paper or other material may be inserted between the screen and the backing electrode if it is desired to print a material other than the backing electrode. 7

As indicated above, it has been customary to use a rotating or oscillating brush to bring the electroscopic powder to the rear surface of the image screen for the purpose of injecting the powder through the screen apertures into the electric field. The brush serves many purposes. It is a mechanical agent to physically bring powder from a reservoir to the rear surface of the screen. It is a charging agent, reacting triboelectrically with the electroscopic powders to effectively charge the powder. And, finally, it acts as a metering device by a complex combination of electrical and mechanical interactions between brush fiber and powder particles. The powder is metered to the screen in a manner which allows effective printing to be done in many cases.

There are many serious problems associated with the use of brush feeds in an electrostatic printing system. The triboelectric reaction that takes place between the brush fibers and the powder particles is very sensitive to and highly dependent upon surface conditions on both the powder particles and the brush fibers, as well as the composition of each. In general, brush materials have proved to be quite sensitive to moisture content in the air and wide variations in the ability to print properly have been noticed with varying humi-dities.

Brush feed systems are highly susceptible to a familiar printing problem known as ghosting, where high-demand areas of copy deplete a saturating type feed system, such as a brush, giving rise to subsequent light and uneven printing in areas of high demand.

In a brush system, the rate of powder release from a brush is independent of the rate of powder delivery to the brush, but the print density is highly dependent on the powder density within the brush itself. Because of the independence of powder release and powder delivery, it is very difficult to control the feed rate to the brush relative to the rate of removal from the brush. This is a fundamental problem in metering and controlling the amount of powder delivered to the system. While this problem, of course, relates to the ghosting problem, it is a more severe and fundamental limitation on any brush feed system.

The powders that are generally used in an electrostatic printing system are thermoplastics and as such are quite 3,273,496 Patented Sept. 20, 1966 sensitive to both heat and pressure with regards to softena continuous printing situation, it has been found that the heat and friction generated at the place of contact between brush and the rear of the image screen causes the build-up of a fused layer of printing powder which gradually, with the passage of time, covers over the open areas of the image screen rendering the printing system unusable. Furthermore, because of the requirement that the brush rub against the screen, in order to release powder, the screen must be made strong enough and must be rigidly supported to resist deformation caused by the applied mechanical pressure of the brush. This also restricts the range of materials from which a screen can be made.

Accordingly, an object of this invention is the provision of a powder delivery system to an image screen in an electrostatic printing system which does not require the use of a brush and therefore eliminates its associated disadvantages.

Another object of the present invention is the provision of a powder delivery system to an image screen whereby an improved print quality and definition over that heretofore available in an electrostatic printing system is achieved.

Yet another object of the present invention is the provision of a powder delivery system wherein no physical contact to any extent is made between the feed system and the rear surface of the image screen allowing the screen to be only so strong as to be able to support its own weight without regard for having to withstand the deforming effects of a contacting feed system.

Still another object of the present invention is the provision of a powder delivery system that will allow the image screen to take any shape independent of the mechanical restrictions imposed on it by a contacting powder delivery system.

Still another object of the present invention is a provision of a novel and useful powder delivery system for an image screen.

These and other objects of the present invention may be achieved by an arrangement wherein powder particles are formed into a cloud, charged, and transported to a region behind the image electrode. There they are acted upon by an electric field established behind the image electrode to accelerate the particles toward the image electrode so that they may pass through the open areas and are accelerated toward the substrate by the electric field between the image electrode and the receiving electrode. The powder particles are then deposited upon the substrate or the receiving electrode to form the desired image.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a diagram illustrating a powder feed system in accordance with this invention;

FIGURE 2 is a front view of the powder feed head employed for feeding powder in accordance with this invention;

FIGURE 3 is a plan view of a powder feed head shown in FIGURE 2; and

FIGURE 4 is a schematic drawing of an alternative powder delivery system in accordance with this invention.

FIGURE 1 is a drawing illustrating how an embodiment of this invention as it is used in a continuous printing system. In the arrangement shown, the image screen 10, which has openings therethrough forming a desired image pattern, is in the form of a continuous loop which is stretched over two supporting rollers respectively 12, 14. These rollers may be driven by a suitable means, not shown. A back plate 16 is provided as the opposite electrode to the image screen. It and the image screen are connected to a suitable source of potential 17 for establishing an electric field therebetween. An image receiving web 18 is passed between the image screen It) and the back plate 16 employing any suitable arrangement, such as pay-out and pick-up rolls (not shown), in synchronism with the moving belt 10. A spacer 19 may be used to prevent contact between the screen and web. Powder particles that come through the image screen openings are thus deposited on the moving web to form the image established by the pattern of openings in the image electrode.

As thus far generally described, the apparatus provided is known and shown, for example, in US. Patent No. 3,081,698.

A powder delivery system in accordance with this invention includes a fluidized powder reservoir 20. This can constitute a chamber in which a supply of electroscopic powder is maintained in what is known as a fluid state. Powder in bulk form can be fluidized by many well-known techniques, including passing low pressure air through a porous membrane on which the powder rests in combination with a source of vibration or other such means. Powder in a fluidized state can be precisely meterd and controlled using various known techniques. The fluidized powder reservoir is connected through a pipe 24 to a suitable powder metering device 26, which may be for instance, a screw feed device and thence through a pipe 28 to the powder feed head 30.

A source of air pressure 32 may be used to provide air through a pipe 34 for the fluidized powder reservoir. Its primary function however is to provide air flow through a pipe 36 to the head 30.

FIGURE 2 shows a front view and FIGURE 3 a plan view of the powder feed head. The view in FIGURE 1 is a side view. The dotted lines in FIGURES 2 and 3 show the internal passageways of the powder feed head. Air under pressure from pipe 36 is fed into an inspirator structure 40.

The inspirator structure 40 is a well-known device and consists simply of a nozzle 42 through which high pressure air is discharged to form a jet creating a low pressure area around the point of discharge. The pipe 28 from the metering section of the powder reservoir terminates in the region of low pressure in the inspirator 40 so that powder metered from the fluid bed is sucked into the inspirator where it mixes with air from the nozzle 42, creating in this region of high turbulence a finely dispersed powder cloud.

Because of the high turbulence within the inspirator, the powder in the cloud makes many contacts with the walls of the inspirator. Through these many contacts, the powder cloud acquires a high triboelectric charge. The walls may be made of a suitable plastic or other material for producing or assisting in the triboelectric charging operation.

The highly charged powder cloud passes out of the inspirator through a short coupling pipe 44 to a powder feed nozzle 46.

The function of the powder feed nozzle 46 is to shape the powder cloud into an even curtain so that powder will be fed in an even manner over the width of the desired printing area. The design of the powder feed nozzle is predicated on the desired width of the curtain that is required for each particular printing application, and, of course, there is no one basic design that will suit all needs. An illustration of a successful design for printing a threeinch width is shown in FIGURE 2. This nozzle essentially provides a simple transition from the one-quarter inch diameter cylindrical shape of the pipe 44 through a transition opening 48 to a one-thirty-second by one inch slit. As powder comes out of this slit, it hits a deflector 50 which turns the powder parallel to the direction of the moving web and causes the powder curtain to spread through another transition opening 52 from one inch to the required three inches.

The air flow which carries the powder is substantially parallel to the image electrode so that air is not forced through the openings of the image electrode. If air were forced through the openings, a blurred print would result.

Since the air flow is parallel to the image electrode, means must be provided for separating the powder from the air stream and forcibly projecting it up, in this case, to the image electrode. This is accomplished by the use of electrodes 54 in the form of parallel wires which are placed adjacent the opening of the transistion opening 52. The wires are supported from the deflector 50, in a plane parallel to that of the image screen. A potential source 56 biases the electrodes 54 relative to the screen to establish an electric field therebetween with a polarity such that the powder particles in the powder cloud emitted from the opening 52 is directed toward the surface of the image screen 10.

As a result of the above-described operation, the moving image screen receives a powder particle spray across its entire width as it moves. In those areas where there are image openings, powder passes through to the electric field established between the image screen and back plate and is deposited on the web 18 in the form of a powder image. In those regions of the image screen where there are no image openings, powder is deposited. This powder must be removed or it will build up to such an extent as to interfere with the successful printing operation. A vacuum scavenging head 58 is employed to remove powder from the screen continuously so that no powder build-up can occur. This head simply comprises a nozzle section 60 which extends from the region of the screen to a collecting chamber 62. There is a filter 64 in the base of the chamber which permits air from a vacuum source 66 to pass through, but prevents powder from being drawn into the vacuum source. Powder collected in the chamber 62 may be returned to the fluid powder reservoir for re-use. Rollers 68, 70 are mounted on top of the scavenging head 58 to support the screen, to prevent it being held or lifted by the suction provided by the scavenging head.

While the embodiment of the invention is shown with a moving screen, this should not be construed as a limitation on the invention. The screen may be stationary, and the feed and scavenging heads can be moved across the screen in the performance of the printing operation without departing from the spirit and scope of this invention.

FIGURE 4 shows another arrangement, in accordance with this invention, for using an air powder cloud for applying powder to the back side of a screen used in an electrostatic printing system. A powder cloud may be produced in the manner described in FIGURE 1, by passing powder derived from a fluidized powder reservoir through a metering device and then an inspirator. The powder cloud is then applied to an opening 70 in one wall of what may be termed a powder cloud drift chamber. This comprises a hollow container 72 having an opening 70 in one wall and an opening 74 in an opposite Wall. A primary screen 76 is placed across an opening in the wall 77 of the chamber which is adjacent the walls containing the openings 70, 74. Spaced from the primary screen is an image screen 78. A back plate 80 is biased positively with respect to the image screen 78 for establishing an electric field therebetween. The substrate 82 is placed by means, not shown, between the space between the image screen 78 and the backing plate 80. A spacer 84 may be employed, if required, to maintain the spacing between the web and the screen 68 at some predetermined minimal distance.

The walls of the drift chamber including screen 76 are conductive and are all maintained at the same electrical potential being connected to a bias source 86. Accordingly, there is within the chamber itself a region free of any electric field. Because the region within the chamber is field free, the powder cloud can permeate the entire space within the chamber in a manner controlled only by the air currents within the chamber, as there is no electric field to influence the charged powder particles. Because of the low resistance to the motion of the air within such an open chamber, the powder cloud spreads throughout the chamber in an even manner and drifts gently through the openings of the screen 76 in an even cloud over the entire area of the image screen 78.

As soon as the powder cloud passes through the screen 76, it comes into a region of high electric field created by the potential difference between walls of the powder cloud drift chamber and the image screen. This electric field causes the powder particles to be moved toward the image screen, to pass through openings corresponding to image areas desired to be printed into the electric field between image screen and back plate to be thus deposited on the web '82,. It is necessary that means he provided for letting the air that carries the powder cloud through the coarse screen escape to a collection chamber, not shown, so that the region of space between the coarse screen and the image electrode is always maintained at essential-1y atmospheric pressure to ensure that no air passe-s through the image openings. If this were not done, print quality would be impaired by the moving air through the image screen.

It should be appreciated that the powder cloud drift chamber may be made in any shape to conform to different shapes of image electrodes. Handling the powder cloud in the manner described provides a very powerful technique in printing over whole areas at once onto irregular shaped substrates, such as curved or irregularly shaped containers of glass or plastic that would prove difficult to handle using conventional techniques. The use of such a feed system allows the use of a very delicate image screen, such as is produced if the screen is formed by various electro-deposition techniques. It would be impossible to use such screens with a brush system as they would not have the strength to withstand the continual pressures exerted by a brush.

In the embodiment of the invention just mentioned, powder that does not pass through the image openings collects on the rear surface of the image screen and primary screen and must be removed after each print. This may be done by passing a high volume of air through the drift chamber, allowing it to carry the deposited powder through the open passages to a collection chamher. The powder can then be returned to the source of fluid bed as was described in the previous example. Instead of printing being done in the continuous fashion as was described before, this device may be used in a cycle consisting of first printing and then scavening. It can be appreciated, however, that this device may be mounted on a moving conveyor so that it can follow an object to be printed during the print cycle and then upon completion of the print cycle be separated, carried back by conveyor to receive another object, and while in transit from the point of separation, the scavenge cycle can take place, so that even though prints are being made discontinuously, objects can be handled in a continuous fashion.

The mechanism of the embodiment of the invention shown in FIGURE 4 is basically similar to that shown in the embodiment of the invention shown in FIGURE 1. Both make use of an electric field behind the image electrode to separate highly charged powder particles from a powder cloud and to project the powder toward the image screen through which the powder particles pass in open areas corresponding to the desired image to be produced.

There has accordingly been shown and described herein a novel, useful and improved arrangement for apply- 6 ing powder particles to the back side of the image screen for the purpose of electrostatic printing.

What is claimed is:

1. In an electrostatic printing system of the type wherein there is provided an image electrode and a back plate positioned adjacent one surface of said image electrode for establishing an electric field therebetween, and electroscopic powder particles are applied to the opposite surface of said image electrode for passing through the image apertures therethrough into said electric field, means for applying said powder particles to said opposite surface of said image screen comprising conductive walls defining a powder cloud chamber, an input opening in one wall of said powder cloud chamber, an output opening in a second wall of said powder cloud chamber opposite said one wall, means for applying a powder cloud to said input opening of said powder cloud chamber to traverse said powder cloud chamber to the output opening, an

opening in a third wall of said powder cloud chamber.

which is adjacent to said one and said second walls, a plurality of spaced wires extending over said opening in said third wall and electrically connected to said chamber walls, means for positioning said powder cloud chamber and said screen adjacent said opposite surface of said image screen, and means for biasing said plurality of spaced wires relative to said image screen for moving powder particles which have passed through said plurality of spaced wires towards said opposite surface of said image screen.

2. In an electrostatic printing system having a conductive image screen, a conductive backing electrode spaced opposite from one surface of said image screen, means for applying a potential to said image screen and backing electrode for establishing an electric field therebetween, means for applying electroscopic powder particles to the other surface of said image screen having a particle size smaller than an image screen opening to pass through the image openings of said image screen, and to be carried by the electric field toward said backing electrode, the improvement in said means for applying powder particles to said other surface of said image screen comprising conductive wall means establishing a substantially electric field free chamber, said wall means being positioned spaced from said other surface of said image screen, said wall means including openings in the side thereof which is opposite said other surface of said image screen, means for establishing a cloud of charged powder particles within the chamber established by said wall means, and means for applying potential between said wall means and said image screen for establishing an electric field therebetween for directing powder particles which pass through the openings in said wall means toward said other side of said image screen.

3. In an electrostatic printing system as recited in claim 2 wherein said wall means including openings has the form of a wire grid.

4. In an electrostatic printing system having a conductive image screen, a conductive backing electrode spaced opposite from one surface of said image screen, means for applying a potential to said image screen and backing electrode for establishing an electric field therebetween, means for applying electroscopic powder particles to the other surface of said image screen having a particle size smaller than an image screen opening to pass through the image openings of said image screen, and to be carried by the electric field toward said electrode, the improvement in said means for applying powder particles to said other surface of said image screen comprising a wire grid positioned adjacent said other surface of said image screen, means for applying a potential between said wire grid and said other surface of said image screen for establishing an electric field therebetween, means including said wire grid for establishing an electric field free powder cloud drift chamber, means for introducing a cloud of charged powder particles into said powder cloud drift chamber whereby powder particles image electrode.

References Cited by the Examiner UNITED STATES PATENTS Wintermute. Carlson. Landrigan et a1. Walkup.

8 Hayford. Fauser et al. Hayford. Epstein. Watson.

Mott et 21. Childress et al. Berry.

ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner. 

1. IN AN ELECTROSTATIC PRINTING SYSTEM OF THE TYPE WHEREIN THERE IS PROVIDED AN IMAGE ELECTRODE AND A BACK PLATE POSITIONED ADJACENT ONE SURFACE OF SAID IMAGE ELECTRODE FOR ESTABLISHING AN ELECTRIC FIELD THEREBETWEEN, AND ELECTROSCOPIC POWER PARTICLES ARE APPLIED TO THE OPPOSITE SURFACE OF SAID IMAGE ELECTRODE FOR PASSING THROUGH THE IMAGE APERTURES THERETHROUGH INTO SAID ELECTRIC FIELD, MEANS FOR APPLYING SAID POWDER PARTICLES TO SAID OPPOSITE SURFACE OF SAID IMAGE SCREEN COMPRISING CONDUCTIVE OPPOSITE DEFINING A POWER CLOUD CHAMBER, AN INPUT OPENING IN ONE WALL OF SAID POWDER CLOUD CHAMBER, AN OUTPUT OPENING IN A SECOND WALL OF SAID POWER CLOUD MEMBER OPPOSITE SAID ONE WALL, MEANS FOR APPLYING A POWDER CLOUD TO SAID INPUT OPENING OF SAID POWDER CLOUD CHAMBER TO TRAVERSE SAID POWDER CLOUD CHAMBER TO THE OUTPUT OPENING, AN OPENING IN A THIRD WALL OF SAID POWDER CLOUD CHAMBER WHICH IS ADJACENT TO SAID ONE AND SAID SECOND WALLS, A PLURALITY OF SPACED WIRE EXTENDING OVER SAID OPENING IN SAID THIRD WALL AND ELECTRICALLY CONNECTED TO SAID CHAMBER WALLS, MEANS FOR POSITIONING SAID POWDER CLOUD CHAMBER AND SAID SCREEN ADJACENT SAID OPPOSITE SURFACE OF SAID IMAGE SCREEN, AND MEANS FOR BIASING SAID PLURALITY OF SPACE WIRE RELATIVE TO SAID IMAGE SCREEEN FOR MOVING POWDER PARTICLES WHICH HAVE PASSED THROUGH SAID PLURALITY OF SPACED WIRES TOWARDS SAID OPPOSITE SURFACE OF SAID IMAGE SCREEN. 